This invention relates to the production of transgenic plants expressing mammalian proteins which function as antigens.
It further relates to methods for suppressing or reducing the immune response of a mammal to an antigen by oral administration of plants expressing mammalian proteins which function as antigens or parts or derivatives of such plants.
Systemic immunosuppressive therapy in autoimmune disease and transplantation is associated with increased rates of infection, malignancy and numerous side effects. The induction of antigen-specific hyporesponsiveness without drugs would therefore be desirable. Immune responses to orally administered proteins are intrinsically modulated and may induce a state of systemic hyporesponsiveness termed oral tolerance (Kay et al., (1989), Immunology, vol.66, pp. 416-421; Peng et al., (1990), Clin. exp. Immunol., vol. 81, pp. 510-515; Lamont et al., (1989), Immunology, vol. 66, pp. 595-599). Although many factors have been implicated in this phenomenon, including soluble mediators and suppressor T cells, it is apparent that antigen processing by mucosal tissue is critical for this effect.
Various studies have been reported of oral administration of antigens thought to be associated with autoimmune diseases, in an effort to induce oral tolerance and prevent or reduce the autoimmune disease.
Oral tolerance to autoantigens has been shown to attenuate experimental induced allergic encephalitis (EAE), adjuvant arthritis (AA), collagen-induced arthritis (CIA) and experimental autoimmune uveoarthritis (EAU) (reviewed in Thompson et al., (1990), Immunology Today, vol. 11, pp. 396-399). The ingestion of myelin basic protein (MBP) during EAE disease has altered its severity, and in recent clinical trials of patients with multiple sclerosis, patients who received MBP had fewer clinical exacerbations during the study period and had reduced numbers of MPB reactive T cells in peripheral blood (Weiner et al., (1993), Science, vol. 259, pp. 1321-1324).
In International Patent Application Publication No.
WO 92/07581, and in Weiner et al. (1992), Proc. Natl. Acad. Sci. USA, vol. 89, pp. 7762-7766, Weiner et al. describe suppression of the mammalian response to allografts by oral administration of splenocytes or splenocyte preparations from tissue donors or of short synthesised peptides corresponding to fragments of class II Major Histocompatibility Complex (MHC) proteins.
There are, however, several problems associated with this approach. Firstly, the complexity of foreign peptide presentation in transplantation makes it difficult to screen peptide sequences suitable for induction of tolerance.
Secondly, the induction of oral tolerance to antigens is dose dependent and insufficient oral antigen may prime gut lymphocytes and cause the opposite and undesired effect of sensitisation. It is therefore necessary to be able to obtain the antigens in large quantities.
Thirdly, the nature of the peptide itself may cause increased rather than reduced immune responsiveness.
If whole antigen proteins are used to induce oral tolerance, there is a greater array of potentially tolerance-inducing peptides presented to the immune system. If complex antigens such as MHC proteins or other transplantation antigens are to be used as intact proteins, however, it is difficult to obtain these proteins in sufficient quantities by in vitro synthesis.
The approach of the present invention to overcoming these problems, not suggested by any of the previously mentioned references, is the expression of appropriate mammalian antigens, for example transplantation antigens or autoantigens, in plants and the administration of these plants or plant materials derived from these plants to a mammal to produce oral tolerance to the expressed mammalian antigens in order to control or suppress allograft rejection or autoimmune responses in the mammal.
Transgenic plants have been used to express a variety of single chain heterologous polypeptides with considerable success (Trudel et al., (1992), Plant Science, v. 87, pp. 55-67). More complex multi-chain proteins such as antibodies have been expressed with less consistent results (Swain, W. F. (1991), Tibtech, v. 9, p. 107).
It has been proposed to express viral antigens in plants with the hope of providing an xe2x80x9cedible vaccinexe2x80x9d, whereby ingestion of plants containing the viral antigen by a human would stimulate an increased immune response and provide immunisation against the virus (Mason et al., (1992), Proc. Natl. Acad. Sci. USA, vol. 89, pp. 11745-11749).
No previous references propose the reduction or suppression of the immune response of a mammal by the administration to the mammal of plants or plant material expressing an appropriate mammalian antigen.
In accordance with one embodiment of the invention, novel plasmids have been constructed for introduction of DNA sequences encoding mammalian antigens such as transplantation antigens or autoantigens into suitable plants.
In accordance with a further embodiment of the invention, novel transgenic plants are provided having inserted into their genome DNA sequences encoding mammalian antigens such as transplantation antigens or autoantigens, these plants being able to express the inserted DNA sequences and produce the antigens encoded therein.
In accordance with a further embodiment of the invention, a method is provided for suppressing or reducing the immune response of a mammal to an antigen, comprising administering orally or enterally to the mammal an effective dose of plant tissue containing an effective amount of the antigen, the plant tissue being obtained from a transgenic plant having in its genome a heterologous expressible gene for the antigen.
In accordance with a preferred embodiment of the invention, a method is provided for controlling or suppressing an immune response of a mammal to an allograft comprising administering orally or enterally to the mammal an effective amount of plant tissue in which is expressed at least one transplantation antigen specific to the allograft whereby the immune response to the allograft is controlled or suppressed.
In accordance with a further preferred embodiment of the invention, a method is provided for controlling or suppressing an autoimmune response of a mammal to an autoantigen comprising administering orally or enterally to the mammal an effective amount of plant tissue in which is expressed the autoantigen whereby the autoimmune response is controlled or suppressed.